LONDON — In a major doping crackdown stretching back eight years, 31 athletes in six sports could be barred from competing in the Olympics after they were caught in retesting of drug samples from the 2008 Beijing Games, and other positive cases could emerge from the 2012 London Games.

The International Olympic Committee opened disciplinary proceedings Tuesday against the 31 unidentified athletes from 12 countries who competed in Beijing and were planning to take part in the Rio de Janeiro Games in August.

“This is a powerful strike against the cheats,” said the IOC president, Thomas Bach. “They show once again that dopers have no place to hide.”

The IOC said it also planned to reanalyze drug tests from the 2014 Sochi Winter Games after allegations samples were tampered with as part of a state-sponsored Russian doping program.

The positive cases from Beijing emerged from the recent retesting of 454 doping samples with “the very latest scientific analysis methods,” the IOC said.

The Olympic body stores samples for 10 years to allow for retesting with improved techniques, with athletes caught facing retroactive disqualification and loss of any medals.

The IOC said it couldn’t immediately identify the athletes caught in the Beijing retests for legal reasons, and it would inform the relevant national Olympic committees soon.

“All those athletes infringing anti-doping rules will be banned from competing at the Olympic Games” in Rio, the IOC said after a teleconference meeting of its policy-making executive board.

Results of retesting of 250 samples from the London Olympics will be announced shortly, the IOC said. Those tests were also aimed at athletes planning to compete in Rio.

The IOC said it also would undertake a “wider retesting program” of medalists from both the Beijing and London Games. Samples of athletes who could be promoted to medals following disqualification of drug cheats also will be retested.

The IOC also asked the World Anti-Doping Agency to launch a “fully-fledged investigation” into allegations that the drug-testing system in Sochi was subverted by Russian officials.

The IOC said it would ask the Lausanne anti-doping lab and WADA to proceed with analyzing Sochi samples “in the most sophisticated and efficient way possible.”

But it’s unclear how many samples are still intact for reliable retesting. Grigory Rodchenkov, the former head of the Russian anti-doping laboratory, told the New York Times last week that he gave Russian athletes a cocktail of drugs before the Olympics and switched tainted urine samples for clean ones during the games. He has offered to assist in retesting.

WADA said Tuesday it has written to Rodchenkov and offered to meet him “immediately” in Los Angeles. No date has been set.

WADA said it also asked a Russian whistleblower, Vitaly Stepanov, for a copy of the recordings of his conversations with Rodchenkov.

The WADA president, Craig Reedie, said the agency is “fully committed’ to investigating the Sochi allegations. The inquiry will be led by WADA’s chief investigator, Mathieu Holz, a former French policeman and Interpol agent, and include independent scientists.

The statute of limitations for retesting was extended in 2015 from eight to 10 years, meaning the Beijing samples remain valid through 2018.

It’s not the first time samples from Beijing have been retested. A few months after those games, the IOC reanalyzed nearly 1,000 of the total of 4,000 samples with a new test for the blood-boosting drug CERA. Five athletes were caught, including the 1,500-meter gold medalist, Rashid Ramzi of Bahrain.

Nearly 500 doping samples from the 2006 Winter Olympics in Turin have been retested. The IOC has not disclosed whether those retests produced any positive cases.

Five athletes were caught in retests of samples from the 2004 Athens Olympics, including the men’s shot put winner, Yuriy Bilonog of Ukraine.